1. Field of the Invention
The present invention relates to a multi-core fuel rod for research reactor and a manufacturing method thereof and, more particularly, to a multi-core fuel rod for research reactor in which monolithic fuel cores made of uranium-molybdenum alloy are disposed in an aluminum (Al) matrix in the form of multi-core, and a manufacturing method thereof.
2. Description of the Prior Art
A large amount of radiations and heat are generated when uranium undergoes nuclear fission. A power reactor utilizes heat generated from the nuclear fission and a research reactor utilizes radiations generated from the nuclear fission. A nuclear fuel is a material used for the nuclear fission. Generally, the research reactor has used highly enriched uranium alloy with uranium content above 90% as a nuclear fuel to obtain high neutron flux for an effective research. However the highly enriched uranium may increase a danger of nuclear proliferation, and thereby low enriched uranium alloy as a nuclear fuel has been developed since 1978 to replace the highly enriched uranium under the leading role of the United States. The main purpose of the development is to solve the problem by lowering the enrichment through the development of high-density nuclear fuel enabling high loading of uranium.
Metal matrix dispersion fuel has been developed by dispersing uranium silicide (U3Si or U3Si2) in an Al matrix. The uranium silicide is a uranium alloy, which has relatively high uranium density and excellent stability in nuclear irradiation. The dispersion fuel is manufactured with fuel material of a uranium alloy type in a powder form mixed with heat carrier such as aluminum having high thermal conductivity, which keeps the fuel rod at low temperature. Since the late 1980's, highly enriched UAlx fuel has been replaced with low enriched fuel of uranium silicide, and the dispersion fuel using U3Si2 dispersed in Al matrix, as a nuclear fuel, has enabled successful development of a new research reactor which is allowed to increase fuel loading up to 4.8 gU/cc.
A high performance research reactor requires high-density fuel, and researches for the high performance fuel had been carried out continuously. However researchers faced the problems that high-density fuel could not be manufactured satisfactorily and fuel reprocessing was not easy. Accordingly, another research was started to survey a material having higher density of uranium than uranium silicide fuel and enabling easier reprocessing. Since the late 1990's, U-Mo alloy fuel has been intensively developed among various applicable fuel materials, because the U-Mo alloy fuel can be manufactured in high density and has excellent stability in the nuclear reactor.
A stepwise irradiation test has been carried out to evaluate the performance of U-Mo fuel. A good result is obtained when the irradiation test is performed in a low power operation, however a problem of fuel damages arises when the irradiation test is performed in a high power operation. In the case of high power operation, the temperature of fuel goes up high by a rapidly increased reaction between aluminum and uranium, and pores and intermetallic compound of UAlx are formed. The pores and low-density UAlx increase the volume of fuel, and cause swelling of fuel. The pores and UAlx having lower thermal conductivity further accelerate the temperature rise and swelling of fuel. Excessive swelling of fuel directly causes fuel damages. The reaction between aluminum and uranium is accelerated as the reaction surface area is being increased. FIG. 1 is a photo of U-Mo dispersion fuel taken after irradiation test, which shows U-Mo fuel particles, UAlx reaction layers, and aluminum matrix. Regardless of the fuel particle size, the thickness of each UAlx layer is almost identical, and the volume of UAlx is increasing as the reaction surface area is increasing. Therefore the reaction surface area should be reduced, because the increase of UAlx causes high temperature and swelling of fuel.
Generally, monolithic fuel is more interested than a dispersion fuel, because the monolithic fuel has a smaller reaction surface area than the dispersion fuel. Nuclear fuel for research reactor may be classified into plate type and rod type, and an irradiation test for U-Mo monolithic fuel of the plate type has been carried out by U.S. ANL with a good result. Development of rod type monolithic fuel is proceeding in Korea, and the monolithic fuel rods of ring or tube type have been suggested. The above fuel has a disadvantage that reaction surface temperature is high in the case of increased uranium loading. The fuel has an advantage that uranium content can be controlled due to discontinuity in the longitudinal direction, however the fuel has a difficulty in manufacturing.
A multi-core fuel rod for research reactor in accordance with the present invention has been developed by locating monolithic fuel with uranium-molybdenum alloy into a fuel rod in a multi-core form, in order to overcome the disadvantages in the conventional fuel rod. The present invention provides high stability of nuclear fuel by significantly reducing the formation of intermetallic compound between uranium alloy and aluminum metal.